Facioscapulohumeral muscular dystrophy (FSHD, FSHMD or FSH) also known as Landouzy-Dejerine muscular dystrophy is an autosomal dominant muscle disorder affecting about 1/20,000 births. It is characterised by progressive weakness and atrophy of the muscles from the face, the upper-arms and shoulder girdle to the lower limbs.
FSHD is genetically linked to contractions of the D4Z4 repeat array on the 4q35 subtelomeric region. Non-affected individuals typically have between 11-100 copies of the 3.3-kb D4Z4 element, while FSHD patients only have 1-10 copies. A typical feature associated with the genetic defect is a decrease in DNA methylation of the contracted D4Z4 array as compared to non-affected individuals (van Overveld et al. 2005 (Ann Neurol. 58: 569-76.)). Whereas a small group of patients with a typical FSHD phenotype does present more than 10 copies of the D4Z4 element, their DNA methylation level is low, similarly to that found in contracted D4Z4 arrays. DNA hypomethylation is typically associated with an open chromatin structure suitable for transcription (de Greef et al. 2009 (Hum Mutat. 30: 1449-59)).
Gabriëls et al. 1999 (Gene 236(1): 25-32) identified the double homeobox 4 (DUX4) gene within each D4Z4 element repeated in the array. The DUX4 sequence was later corrected as published by Kowaljow et al. 2007 (Neuromuscul Disord 17: 611-23) and is available under the NCBI Genbank accession number: AF117653.2. Subsequent studies showed that the encoded DUX4 protein was expressed in primary myoblasts and biopsies of patients with FSHD but not in non-affected individuals, and that the DUX4 protein is a transcription factor targeting a large set of genes including inter alia genes encoding further transcription factors, and that DUX4 gene activation at the FSHD locus initiates a transcription cascade leading to muscle atrophy, inflammation, decreased differentiation potential and oxidative stress, recapitulating the key features of FSHD (Bosnakowski et al. 2008 (EMBO J 27(20): 2766-79); Kowaljow et al. 2007 (Neuromuscul Disord 17: 611-23); Dixit et al. 2007 (Proc Natl Acad Sci USA 104: 18157-18162)). Double homeobox 4 is thus considered a major contributor to the pathology of FSHD muscles.
Dixit et al. 2007 (supra) also demonstrated in myoblast cultures that whereas transcription can initiate at any D4Z4 element within the repeat array, a prevalent stable DUX4 mRNA originates from the most distal D4Z4 unit and extends into the pLAM region which flanks the telomeric side of the D4Z4 array, whereby the pLAM region provides the DUX4 transcript with an intron and a polyadenylation signal (FIGS. 1 and 2). However, also additional transcripts were identified that span several D4Z4 units, may have various parts spliced out, and may also comprise the pLAM region (Snider et al. 2009. Hum Mol Genet 18: 2414-30; Coppée et al., unpublished, see Figures 26-28). Moreover, polymorphisms have been found in the pLAM region such as the presence or absence of a 1.6 kb sequence within its intron (Gabriëls et al. 1999, supra; van Deutekom et al. 2009. Hum Mutat 30: 1449-59).
Lemmers et al. 2010 (Science, August 19) propose a unifying genetic model for FSHD.
Furthermore, the homologous DUX4c gene was identified 42 kb centromeric of the D4Z4 array, within a truncated and inverted solitary D4Z4 unit. The DUX4c gene encodes a 47-kDa protein with a double homeodomain identical to DUX4 but divergent in the carboxyl-terminal region. The DUX4c protein is expressed at low levels in control muscles, it is induced in muscles of patients affected with Duchenne muscular dystrophy and is present at similar or yet higher levels in FSHD muscles. Additional experiments suggested that DUX4c could be involved in myoblast proliferation during muscle regeneration and that changes in its expression could contribute to the FSHD pathology (Ansseau et al. 2009 (PLoS One 4(10): e7482).
In certain tumour types a fusion gene is seen that includes the 3′ region of the DUX4 gene as a result of chromosome rearrangements. Fusion between CIC, a human homolog of Drosophila capicua, and DUX4 was seen in Ewing's family tumours (EFTs) (Kawamura-Saito et al. 2006. Hum Mol Genet 15: 2125-2137) and paediatric undifferentiated soft tissue sarcomas (USTS) (Yoshimoto et al. 2009. Cancer Genet Cytogenet 195: 1-11), and rhabdomyosarcomas (RMS) showed fusion between the EWSR1 gene and DUX4. (Sirvent et al. 2009. Caner Genet Cytogenet 195: 12-08). As a consequence of fusion with the C-terminal fragment of DUX4 the resulting fusion proteins acquire an enhanced transcriptional activity, which leads to tumour formation.